1. Technical Field
The present invention relates to composite pigments for use in paper coatings. The composite pigments are also useful as paper fillings, and in paint, ink, rubber, and plastic compositions. The present invention also relates to a process for the production of the composite pigments.
2. Description of the Prior Art
Structured or high-bulking pigments have been shown to offer improvements in optical and printing properties when used as a partial or total substitution for clay in paper coatings. The first of the high-bulking pigments introduced into paper coatings was calcined clay. By virtue of its internal pore volume and its disruption of tight parallel clay packing, experienced with standard clay in paper coatings, improved porosity and light scatter was obtained. Calcination of clay, however, produces a product which has high abrasion and which is deleterious to the wires used in the paper making processing. In addition, the use of calcined clay at higher levels in paper coatings may result in either runnability problems or reduced gloss.
More recently, chemically structured kaolin products have been found useful as functional fillers, as titanium dioxide extenders, and as reinforcing agents for paper, paint, rubber and plastics. These chemically structured products include synthetic alkali metal alumino-silicates (U.S. Pat. No. 4,933,387) obtained by hydrothermal modification of kaolin with alkali metal silicates; structured clay obtained by reaction of fine particles of kaolin with an organic silicon compound such as tetramethoxy silane or tetraethoxy silane (U.S. Pat. No. 4,818,294); high bulking kaolin pigments obtained by flocculating kaolin clay with a water-soluble cationic polyelectrolyte (U.S. Pat. Nos. 4,738,726 and 4,767,466); and a bulked kaolin pigment obtained by reacting a low molecular weight polyamine with kaolin (U.S. Pat. No. 4,078,941). Additionally, U.S. Pat. No. 4,640,716 teaches the preparation of a structured pigment formed by the reaction of zirconium ion in the absence of a binder at a pH of 2.8-3.9 with an uncalcined kaolin clay.
These products exhibit low abrasion while providing improvements in optical properties, printability, and as a result of their increased bulk, improved coating coverage over a substrate or lower coating weight. Several of these products, however, exhibit either poor rheology or lack of stability in standard paper coating procedures. A theology appropriate to blade coating speeds in excess of 4,000 feet per minute is desirable. Additionally, structured pigments must survive the shear conditions encountered in coating preparation and use, such as jet cooking, pigment dispersion, and pumping and blade metering.
The above technologies for preparing structured pigments and their use in paper coatings to reduce the level of expensive opacifying pigments such as titanium dioxide, are now conventional practice in the art. Still, pigments with high opacifying powers such as titanium dioxide continue to be used in paper coatings and fillings. The industry continues to search for less expensive pigments to achieve higher opacity at lower cost.
U.S. Pat. No. 3,726,700 discloses the formation of a composite pigment by adhering titanium dioxide particles to calcined clay. The titanium dioxide particles are adhered to the clay by means of hydrous oxides of aluminum, silicon, and titanium. The hydrous oxides are precipitated from a slurry which also contains the clay and titanium dioxide. One problem is that in this process, both clay and titanium dioxide are capable of participating in homoflocculation (aggregation of multiple particles of either the clay or titanium dioxide, e.g., clay-to-clay or titanium dioxide-to-titanium dioxide) in addition to the desired heteroflocculation. The homoflocculation of titanium dioxide in particular can lead to a drop in light scattering efficiency.
U.S. Pat. No. 4,117,191 discloses a composite silicate pigment prepared by precipitating hydrous metal silicate particles onto the planar surfaces of clay particles. It is indicated in the patent that this results in an improvement in optical efficiency. A disadvantage of the process of this patent is that it provides limited flexibility in terms of the types of materials which can be included within the composite pigment.
U.S. Pat. No. 4,331,706 discloses a composite pigment comprising hydrous zinc oxide deposited on the surface of a core particle, followed by calcination. Co-calcination of comminuted materials such as kaolins in the presence of inorganic binders is also disclosed in U.S. Pat. Nos. 3,853,457; 3,864,140; and 3,856,545. All of these processes require the expensive calcination step.
U.S. Pat. No. 4,820,554 discloses an aggregated pigment containing kaolin and up to 12% calcium carbonate. The aggregated pigment is prepared by chemically reacting particulate kaolin with a metal chloride such as a silicon tetrachloride (SICl.sub.4). This process has the disadvantage that it requires the use of silicon tetrachloride which is toxic and, and produces hydrogen chloride as a by-product, which must be neutralized. The process also requires an aging time for full development of optical properties.
U.S. Pat. No. 3,453,131 discloses the preparation of composite pigments by the use of fatty acid coupling agents. This type of coupling agent restricts use of the composite pigments in non-aqueous formulations or under dry conditions.
South African Applications Nos. 905093 and 905091 disclose a process for making structured aggregate pigments in which two or more non-identical pigments are joined by a silico-aluminate gel as a de-stabilizing agent. This process also suffers from the probability of homoflocculation of ingredient pigments, and is complex in execution.
U.S. Pat. No. 2,176,877 discloses a process for the manufacture of a composite pigment. A primary pigment is dispersed in an aqueous suspension using a first dispersing agent. Separately, a secondary pigment is dispersed in an aqueous suspension using a second dispersing agent. This second dispersing agent is antipathetic to the first dispersing agent. By antipathetic, it is meant that the second dispersing agent is capable of furnishing ions, on admixture of the primary pigment and secondary pigment, which neutralize the dispersing ions of the first dispersing agent. The dispersing agents are all salts, such as aluminum chloride and sodium silicate. One example given is a coflocculation obtained by admixing a suspension of titanium dioxide, prepared by means of the acidic salt, aluminum chloride, with a suspension of barium sulfate prepared, with the basic substance, sodium silicate. Examples of other acidic dispersants given in the patent are hydrogen sulfide and lead chloride. Following coflocculation, the integrated pigment is then separated from the supernatant liquor, washed, dried, and pulverized.